Frame dropping in Vegas Pro 11

I’m using Vegas Pro 11 for a few years, and have dozens of videos created by it without any problem. About a week ago, however, I’ve got an issue that renders me helpless: frame dropping. A few details:
1. Source material. I combine HD video footage with 5.1 audio from my Sony DRV550 video camera with pictures from my Canon T5i. I also add my comments in a separate audio track and an underlying music in yet one more audio track. Same routine for years.
2. Computer: HP Elitebook 8540w, i7 M620 @2.67GHz, 64-bit Win 7 SP1, 320G 5400RPM HD yesterday replaced with 500G 7200RPM one (which didn’t improve the situation).
3. Project settings (again, same for a long time):
3.1. Video: 1920*1080, 29.97 fps, progressive scan, 1.000 0 square pixel, 32-bit pixel format, deinterlace: interpolate fields.
3.2. Audio: 5.1 Surround, 48K, bit depth:16, quality: best, cutoff frequency: 120.
3.3. Dynamic RAM preview: 2G, GPU accel: NVIDIA Quadro FX880M.
3.4. Render to AVCHD 1920*1080-60i 5.1 Surround.
4. Length of a video: 15-20 min (and I rendered longer than an hour videos with these settings).
5. Preview shows everything perfectly.
6. What happens when I watch the video after rendering:
a. Video parts are always rendered without any problem.
b. However, many pictures are either completely dropped (video goes completely black in the playback) or partially dropped (a picture is there for a second, but then it’s replaced by black screen).
c. Pictures that are set in Pan/Crop window to gradually zoom in playback often do zoom, but choppy, that is zooming picture is alternated with black frames back and forth during playback.
d. When I re-run the rendering other pictures are dropped, so the effect seems random and doesn’t depend on the material, but rather on my computer environment.
7. The most embarrassing is that I didn’t change anything on my box, project settings or viewers (I use Window Media Player, VLC and Media Player Classic). If anything, I improved the computer by replacing a slower HD to a faster (and bigger) one.

Yes, I have automatic updates on, and it has to stay this way, unfortunately. Regarding the HD i started having these issues before HD replacement. And hte replacement was done by a special software that produces a logical copy of the disk. ALl other components of the system work fine, and i use tons of software due to the nature of my job.

Thank you anyway, I plan to reinstall Windows in a week due to other reasons. This will require new installation of the Vegas, of course. Hopefully this helps. I'll report the result.